As a material of an adhesive film for FPC (flexible printed circuit board) protective film, various adhesive compositions obtained by combination of thermoplastic resins and epoxy resins as thermosetting resins have hitherto been known (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 9-132710, 9-125037, 5-5085, 3-6280, 2-145676, 62-274690, 60-130666, 1-135844 and 61-43550).
Among adhesive films disclosed in the above-described patent publications, an adhesive film formed from a composition containing a phenoxy resin, an epoxy resin and a curing agent is comparatively superior in performances such as heat resistance, adhesion to metal parts, etc. and, therefore, the adhesive film has hitherto been considered to be useful. It is also disclosed to use a polyester resin or various elastomers in place of a phenoxy resin, as the thermoplastic resin. Furthermore, dicyanamide is often used, comparatively, because it is particularly superior in potentiality.
However, the above-described adhesive composition containing a phenoxy resin, an epoxy resin and dicyandiamide as a curing agent, as a main component, has a drawback in that a dimensional change caused by a stress produced during the thermal curing process is comparatively large. Reduction in such a dimensional change, i.e. improvement of the dimensional stability, is further required in the usage of the adhesive for FPC protective film. In the above-described conventional composition, however, it was very difficult to respond to such a requirement.
On the other hand, it has been known as a conventional technique to improve the nonflammability of the resin composition by adding a combination of a brominated epoxy resin and antimony pentaoxide in the general field of the resin composition. This technique is normally characterized by mixing antimony pentaoxide powder with a resin component to form a nonflammable composition. Since the antimony pentaoxide powder is comparatively inexpensive, its utilization value is high. However, since an average particle diameter of the antimony pentaoxide powder is normally not less than 0.5 .mu.m, sedimentation of the powder is liable to arise due to gravity and it was difficult to obtain a composition in the state where the respective components are uniformly dispersed. That is, in case of forming an adhesive composition containing the antimony pentaoxide powder, sedimentation of the antimony pentaoxide powder is liable to arise in a mixed solution of an active amount of the antimony pentaoxide, a resin component and a solvent. Therefore, it was difficult to realize the state (structure) where the respective components are uniformly mixed. Such a nonuniform structure causes deterioration of the adhesion, which results in drastic deterioration of the performance as the adhesive for FPC protective film.